The present invention relates to a mould additive for continuous casting of steel. When steel is produced by continuous casting, a mould additive is indispensable and the quality of steel depends upon the quality of the mould additive.
When a mould additive is added to the surface of molten steel in a mould, it is fused gradually by the heat from the molten steel, to make three layers consisting of a fused layer, a semifused layer (sintered layer) and a layer of unfused additive.
The characteristics which such mould additive should have are as follows:
(1) heat insulation and oxidation prevention of molten steel surface; PA1 (2) uniform fusibility; PA1 (3) ability to absorb floating substances such as Al.sub.2 O.sub.3 etc.; PA1 (4) ability to lubricate between mould and solid shell of molten steel. PA1 (1) Environmental problems such as the generation of dust or fires when being fed into the mould. PA1 (2) There is a risk of uneven distribution of additive ingredients compared with granular types. PA1 (3) Due to non-uniform fusion and non-uniform influx of the additive to the spaces between the mould and the solid shell of molten steel, it is difficult to perform even lubrication between the mould and the solid shell of molten steel compareed with granular types. As a result, the solid shell is unevenly cooled resulting in higher probability of surface cracks in the cast steel. PA1 (1) Less thermal insulation than powder type additives. PA1 (2) Slower slagging than powdery types, making it unsuitable adoption for high speed casting of steel. PA1 (3) Not applicable for automatic feeders commonly adapted in iron and steel works, because conventional granular type mould additives are fragile, break easily during transportation and have less spreadability in a mould. Therefore, most iron and steel works use a powdery mould additive when they use automatic feeders.
Among those characteristics, oxidation prevention, the ability to absorb floating substances and the lubricating ability are characteristics demanded for the fused layer of mould additive. On the other hand, heat insulation and uniform fusibility are required for the semi-fused layer (sintered layer) and the un-fused layer of mould additive, and these characteristics are greatly influenced by the shape of the mould additive particles.
The shape of conventional mould additive can roughly be divided into three types, that is powdery, granular (columnar shape: average grain size about 1 to 3mm, FIG. 2) and spherical (hollow type), and among them the powdery and granular types are chiefly used.
Powdery types are comparatively more advantageous in heat insulation than granular types and characteristically slag faster due to their large specific surface area. Therefore powdery type mould additive are used mainly for low carbon aluminum-killed steel which is easily affected by contamination defects such as pin-holes and blow holes, as well as for high speed continuous casting where casting speed is at least 1.6m/min in which even speedier slagging and even speedier influx are required.
Granular types are superior from an environmental aspect because they generate less dust, and they also have such merits as uniform fusing of the granular layer, and the uneven distribution of additive ingredients is small so the composition of the slag is also uniform. For these reasons, granular additives are mainly used for medium carbon steels which require uniform fusion and uniform influx of the additive or for use in low speed casting which gives priority to environmental problems.
Hollow spherical types have many superior points environmentally, in fluidity in a mould and in heat insulation of molten steel but there are few examples of use in actual casting units.
Although the three types of mould additives described above have many merits respectively, they also have the following drawbacks.